The research work currently underway covers all aspects of the mesencephalic nucleus of the trigeminal nerve from its origin to its role in the regulation of jaw movements and related reflexes. Embryos of the duck, chick, and quail will be used as experimental models in this study. The quail cells used as biological cell markers in our interspecific transplantation have yielded direct and conclusive evidence for a neural crest origin for the cells of the mesencephalic nucleus of V. Morphological changes in the cells of the nucleus during development are presently being studied at the EM level in correlation with the onset and progress of jaw movements during development under (a) normal and (b) experimental conditions following the administration intravenously of an antithyroid drug.